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Subject: Re: [PATCH RFC 0/3] Static calls
To:     Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>
References: <cover.1541711457.git.jpoimboe@redhat.com>
 <20181109072811.GB86700@gmail.com>
 <CALCETrU3tPPOiuTaLqUy-L4sPvZRU8BLYML-BgQH_LZhA4bzSQ@mail.gmail.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <54b6c3b1-acf3-d591-cd2a-79eae4ff0ebb@rasmusvillemoes.dk>
Date:   Fri, 9 Nov 2018 21:53:06 +0100
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On 09/11/2018 16.16, Andy Lutomirski wrote:
> On Thu, Nov 8, 2018 at 11:28 PM Ingo Molnar <mingo@kernel.org> wrote:
>>
>>
>> All other usecases are bonus, but it would certainly be interesting to
>> investigate the impact of using these APIs for tracing: that too is a
>> feature enabled everywhere but utilized only by a small fraction of Linux
>> users - so literally every single cycle or instruction saved or hot-path
>> shortened is a major win.
> 
> For tracing, we'd want static_call_set_to_nop() or something like that, right?
> 

Hm. IIUC, when gcc sees static_call(key)(...), it has to generate code
to put the right values in %rdi, %rsi etc.. Even if the function is void
(*)(void), gcc would still need to shuffle things around (either spill
and reload, or move %rdi to some callee saved register). So if the
static_call is noop'ed out most of the time, that seems like a net loss?
With an unlikely static_key, gcc can do all the parameter setup and
reloading in an out-of-line chunk of code.

static calls seems like a quite useful concept, but only/mostly if
_some_ function needs to be called at that spot.

Aside: there should be some compile-time check that
static_call_set_to_nop can only be used if the return type is void.

Rasmus